1. Field of the Invention
This invention relates to solar receivers for solar-powered hot gas engines.
2. Description of the Prior Art
Solar receivers typically include an insulated housing defining a receiver cavity and an aperture in the housing to admit solar radiation for conversion to thermal (heat) energy inside the cavity, for subsequent utilization such as by a heat engine of the hot gas (Stirling) type.
Significant heat losses may occur from convection effects with open-aperture solar Stirling receivers. For example, for a 22 cm diameter aperture, 20.degree. C. air entering with a velocity of 1 m/s flowing into 1/3 of the aperture heated to 520.degree. C. and flowing out the remaining 2/3 area of aperture will carry away about 7.75 kW of thermal energy. Typical average wind speed of 5 m/s for the U.S. may cause serious problems in reaching current receiver efficiency estimates of 85%.
Quartz (fused silica) windows mounted in the plane of the aperture have been suggested as possibly providing a method of reducing convection losses, as well as heat (infra-red) radiation losses. The FIGURE is a schematic of a solar-powered Stirling hot gas engine having a receiver including an insulated housing surrounding the hot gas engine heater head with a quartz window mounted in the plane of the aperture. A problem with quartz windows, unfortunately, is reflectivity. For instance, baseline 89.2 m.sup.2 parabolic dish collector-concentrator with 90% reflectance at 0.85 kW/m.sup.2 insolation would be delivering 68.2 kW of radiant energy to the receiver. For a nominal 10% reflective flat quartz window, the reflectivity would cause a 6.8 kW loss, which would be sacrificed before any gain from saved convection losses would be realized.
It has also been suggested by Francia and, later, by Chubb to use an assembly of transparent, open-ended tubes oriented perpendicular to the aperture plane to fill the aperture of a solar receiver (see e.g. Chubb, "Solar Energy" vol. 23, pp. 217-221, 1979, FIGS. 1 and 2). Solar radiation incident on the ends of the tubes at angles to the tube axes would be reflected down the tube and into the cavity. However, this arrangement suffers from back reflection from the ends of the tubes which can constitute a significant fraction of the aperture planar area. Also, the arrangement does not provide a true barrier against ambient wind and convection currents, and can be complicated and expensive to construct.